Clozapine and haloperidol differently suppress the MK-801-increased glutamatergic and serotonergic transmission in the medial prefrontal cortex of the rat.

The administration of noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists such as phencyclidine and ketamine has been shown to increase the extracellular concentration of glutamate and serotonin (5-HT) in the medial prefrontal cortex (mPFC). In the present work, we used in vivo microdialysis to examine the effects of the more potent noncompetitive NMDA receptor antagonist, MK-801, on the efflux of glutamate and 5-HT in the mPFC, and whether the MK-801-induced changes in the cortical efflux of both transmitters could be blocked by clozapine and haloperidol given systemically or intra-mPFC. The systemic, but not the local administration of MK-801, induced an increased efflux of 5-HT and glutamate, which suggests that the NMDA receptors responsible for these effects are located outside the mPFC, possibly in GABAergic neurons that tonically inhibit glutamatergic inputs to the mPFC. The MK-801-induced increases of extracellular glutamate and 5-HT were dependent on nerve impulse and the activation of mPFC AMPA/kainate receptors as they were blocked by tetrodotoxin and NBQX, respectively. Clozapine and haloperidol blocked the MK-801-induced increase in glutamate, whereas only clozapine was able to block the increased efflux of 5-HT. The local effects of clozapine and haloperidol paralleled those observed after systemic administration, which emphasizes the relevance of the mPFC as a site of action of these antipsychotic drugs in offsetting the neurochemical effects of MK-801. The ability of clozapine to block excessive cortical 5-HT efflux elicited by MK-801 might be related to the superior efficacy of this drug in treating negative/cognitive symptoms of schizophrenia.

Antipsychotic drugs reverse the AMPA receptor-stimulated release of 5-HT in the medial prefrontal cortex.

The prefrontal cortex (PFC) is involved in the pathophysiology of schizophrenia. PFC neuronal activity is modulated by monoaminergic receptors for which antipsychotic drugs display moderate-high affinity, such as 5-HT(2A) and alpha(1)-adrenoceptors. Conversely, PFC pyramidal neurons project to and modulate the activity of raphe serotonergic neurons and serotonin (5-HT) release. Under the working hypothesis that atypical antipsychotic drugs may partly exert their action in PFC, we assessed their action on the in vivo 5-HT release evoked by increasing glutamatergic transmission in rat medial PFC (mPFC). This was achieved by applying S-AMPA in mPFC (reverse dialysis) or by disinhibiting thalamic excitatory afferents to mPFC with bicuculline. The application of haloperidol, chlorpromazine, clozapine and olanzapine in mPFC by reverse dialysis (but not reboxetine or diazepam) reversed the S-AMPA-evoked local 5-HT release. Likewise, the local (in mPFC) or systemic administration of these antipsychotic drugs reversed the increased prefrontal 5-HT release produced by thalamic disinhibition. These effects were shared by the 5-HT(2A) receptor antagonist M100907 and the alpha(1)-adrenoceptor antagonist prazosin. However, raclopride (DA D2 antagonist) had very modest effects. These results suggest that, besides their action in limbic striatum, antipsychotic drugs may attenuate glutamatergic transmission in PFC, possibly by interacting with 5-HT(2A) and/or alpha(1)-adrenoceptors.

Clozapine and olanzapine, but not haloperidol, suppress serotonin efflux in the medial prefrontal cortex elicited by phencyclidine and ketamine.

N-methyl-D-aspartate (NMDA) receptor antagonists such as phencyclidine (PCP) and ketamine can evoke psychotic symptoms in normal individuals and schizophrenic patients. Here, we have examined the effects of PCP (5 mg/kg) and ketamine (25 mg/kg) on the efflux of serotonin (5-HT) in the medial prefrontal cortex (mPFC) and their possible blockade by the antipsychotics, clozapine, olanzapine and haloperidol, as well as ritanserin (5-HT2A/2C receptor antagonist) and prazosin (alpha1-adrenoceptor antagonist). The systemic administration, but not the local perfusion, of the two NMDA receptor antagonists markedly increased the efflux of 5-HT in the mPFC. The atypical antipsychotics clozapine (1 mg/kg) and olanzapine (1 mg/kg), and prazosin (0.3 mg/kg), but not the classical antipsychotic haloperidol (1 mg/kg), reversed the PCP- and ketamine-induced increase in 5-HT efflux. Ritanserin (5 mg/kg) was able to reverse only the effect of PCP. These findings indicate that an increased serotonergic transmission in the mPFC is a functional consequence of NMDA receptor hypofunction and this effect is blocked by atypical antipsychotic drugs.

Effects of acute olanzapine after sustained fluoxetine on extracellular monoamine levels in the rat medial prefrontal cortex.

The combination of selective serotonin reuptake inhibitors with atypical antipsychotic drugs exhibits beneficial effects in treatment-resistant depression. We investigated the effects of a 2-week treatment with a low fluoxetine dose (3 mg/kg per day) plus a single injection of olanzapine (3 mg/kg) on the dialysate concentration of noradrenaline, dopamine and serotonin (5-HT) in the medial prefrontal cortex of the rat. Chronic fluoxetine increased only 5-HT levels whereas single olanzapine administration increased the concentration of catecholamines and decreased that of 5-HT to a comparable extent in vehicle- and fluoxetine-treated rats. Therefore, it is possible that the therapeutic benefit of this pharmacological combination may not be associated to changes in the cortical concentration of monoamines, but to postsynaptic blockade of monoaminergic receptors.

The therapeutic role of 5-HT1A and 5-HT2A receptors in depression.

The selective serotonin reuptake inhibitors (SSRIs) are the most frequently prescribed antidepressant drugs, because they are well tolerated and have no severe side effects. They rapidly block serotonin (5-HT) reuptake, yet the onset of their therapeutic action requires weeks of treatment. This delay is the result of presynaptic and postsynaptic adaptive mechanisms secondary to reuptake inhibition. The prevention of a negative feedback mechanism operating at the 5-HT autoreceptor level enhances the neurochemical and clinical effects of SSRIs. The blockade of 5-HT2A receptors also seems to improve the clinical effects of SSRIs. These receptors are located postsynaptically to 5-HT axons, mainly in the neocortex. Pyramidal neurons in the prefrontal cortex are particularly enriched in 5-HT2A receptors. Their blockade may affect the function of prefrontal-subcortical circuits, an effect that probably underlies the beneficial effects of the addition of atypical antipsychotic drugs, which are 5-HT2A receptor antagonists, to SSRIs in treatment-resistant patients.

In vivo efflux of serotonin in the dorsal raphe nucleus of 5-HT1A receptor knockout mice.

In the dorsal raphe nucleus (DR), extracellular serotonin (5-HT) regulates serotonergic transmission through 5-HT1A autoreceptors. In this work we used in vivo microdialysis to examine the effects of stressful and pharmacological challenges on DR 5-HT efflux in 5-HT1A receptor knockout (5-HT1A-/-) mice and their wild-type counterparts (5-HT1A+/+). Baseline 5-HT concentrations did not differ between both lines of mice, which is consistent with a lack of tonic control of 5-HT1A autoreceptors on DR 5-HT release. (R)-(+)-8-Hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT, 0.5 mg/kg) reduced 5-HT levels to 30% of basal values in 5-HT1A+/+ mice, but not in 5-HT1A-/- mice. The selective 5-HT1B receptor agonist 1,4-dihydro-3-(1,2,3,6-tetrahydro-4-pyridinyl)-5H-pyrrolo[3,2-b]pyridin-5-one dihydrochloride (CP 93129, 300 micro m) reduced dialysate 5-HT to the same extent (30-40% of baseline) in the two genotypes, which suggests a lack of compensatory changes in 5-HT1B receptors in the DR of such mutant mice. Both a saline injection and handling for 3 min increased DR dialysate 5-HT in mutants, but not in 5-HT1A+/+ mice. Fluoxetine (5 and 20 mg/kg) elevated 5-HT in a dose-dependent manner in both genotypes. However, this effect was markedly more pronounced in the 5-HT1A-/- mice. The increased responsiveness of the extracellular 5-HT in the DR of 5-HT1A receptor knockout mice reflects a lack of the autoinhibitory control exerted by 5-HT1A autoreceptors.

Co-expression and in vivo interaction of serotonin1A and serotonin2A receptors in pyramidal neurons of prefrontal cortex.

The prefrontal cortex plays a key role in the control of higher brain functions and is involved in the pathophysiology and treatment of schizophrenia. Here we report that approximately 60% of the neurons in rat and mouse prefrontal cortex express 5-HT(1A) and/or 5-HT2A receptor mRNAs, which are highly co-localized (approximately 80%). The electrical stimulation of the dorsal and median raphe nuclei elicited 5-HT1A-mediated inhibitions and 5-HT2A-mediated excitations in identified pyramidal neurons recorded extracellularly in rat medial prefrontal cortex (mPFC). Opposite responses in the same pyramidal neuron could be evoked by stimulating the raphe nuclei at different coordinates, suggesting a precise connectivity between 5-HT neuronal subgroups and 5-HT1A and 5-HT2A receptors in pyramidal neurons. Microdialysis experiments showed that the increase in local 5-HT release evoked by the activation of 5-HT2A receptors in mPFC by DOI (5-HT2A/2C receptor agonist) was reversed by co-perfusion of 5-HT1A agonists. This inhibitory effect was antagonized by WAY-100635 and the prior inactivation of 5-HT1A receptors in rats and was absent in mice lacking 5-HT1A receptors. These observations help to clarify the interactions between the mPFC and the raphe nuclei, two key areas in psychiatric illnesses and improve our understanding of the action of atypical antipsychotics, acting through these 5-HT receptors.

Stimulation of alpha1-adrenoceptors in the rat medial prefrontal cortex increases the local in vivo 5-hydroxytryptamine release: reversal by antipsychotic drugs.

Pyramidal neurons of the medial prefrontal cortex (mPFC) project to midbrain serotonergic neurons and control their activity. The stimulation of prefrontal 5-HT2A and AMPA receptors increases pyramidal and serotonergic cell firing, and 5-hydroxytryptamine (5-HT) release in mPFC. As the mPFC contains abundant alpha1-adrenoceptors whose activation increases the excitability of pyramidal neurons, we examined the effects of their stimulation on local 5-HT release, using microdialysis. The application of the alpha1-adrenoceptor agonist cirazoline by reverse dialysis increased the prefrontal 5-HT release in a concentration-dependent manner, an effect antagonized by coperfusion of TTX, prazosin (alpha1-adrenoceptor antagonist), BAY x 3702 (5-HT1A agonist), NBQX (AMPA/KA antagonist) and 1S,3S-ACPD (mGluR II/III agonist), but not by MK-801 (NMDA antagonist). Cirazoline also enhanced the increase in 5-HT release induced by DOI (5-HT2A/2C agonist) and AMPA. In addition, M100907 (5-HT2A antagonist) but not SB-242084 (5-HT2C antagonist) reversed the cirazoline- and AMPA-induced 5-HT release. These results suggest that the stimulation of prefrontal alpha1-adrenoceptors activates pyramidal afferents to ascending serotonergic neurons. The effect of cirazoline was also reversed by coperfusion of classical (chlorpromazine, haloperidol) and atypical (clozapine, olanzapine) antipsychotics, which suggests that a functional antagonism of the alpha1-adrenoceptor-mediated activation of prefrontal neurons may partly underlie their therapeutic action.

In vivo modulation of 5-hydroxytryptamine release in mouse prefrontal cortex by local 5-HT(2A) receptors: effect of antipsychotic drugs.

In the rat, postsynaptic 5-hydroxytryptamine2A receptors medial prefrontal cortex control the activity of the serotonergic system through changes in the activity of pyramidal neurons projecting to the dorsal raphe nucleus. Here we extend these observations to mouse brain. The prefrontal cortex expresses abundant 5- hydroxytryptamine2A receptors, as assessed by immunohistochemistry, Western blots and in situ hybridization procedures. The application of the 5-hydroxytryptamine2A/2C agonist DOI (100 microm) by reverse dialysis in the medial prefrontal cortex doubled the local release of 5-hydroxytryptamine. This effect was reversed by coperfusion of tetrodotoxin, and by the selective 5-hydroxytryptamine2A receptor antagonist M100907, but not by the 5-hydroxytryptamine2C antagonist SB-242084. The effect of DOI was also reversed by prazosin (alpha1-adrenoceptor antagonist), BAY x 3702 (5-hydroxytryptamine1A receptor agonist), NBQX (alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate/kainic acid antagonist) and 1S,3S-ACPD (mGluR II/III agonist), but not by dizocilpine (N-methyl-d-aspartate antagonist). alpha-Amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate mimicked the 5-hydroxytryptamine elevation produced by DOI, an effect also reversed by BAY x 3702. Likewise, the coperfusion of classical (chlorpromazine, haloperidol) and atypical antipsychotic drugs (clozapine, olanzapine) fully reversed the 5-hydroxytryptamine elevation induced by DOI. These observations suggest that DOI increases 5-hydroxytryptamine release in the mouse medial prefrontal cortex through the activation of local 5-hydroxytryptamine2A receptors by an impulse-dependent mechanism that involves/requires the activation of local alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate receptors. This effect is reversed by ligands of receptors present in the medial prefrontal cortex, possibly in pyramidal neurons, which are involved in the action of antipsychotic drugs. In particular, the reversal by classical antipsychotics may involve blockade of alpha1-adrenoceptors, whereas that of atypical antipsychotics may involve 5-hydroxytryptamine2A receptors and alpha1-adrenoceptors.